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Author Topic: Squares not square.  (Read 3201 times)

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Squares not square.
« on: June 19, 2013, 09:45:31 PM »
So I have a nice home built 3 axis cnc router. I draw up in turbocad to lazycam then to Mach 3.
Every now and then when I want to cut a bunch of squares it starts to cut the squares then when it gets to the corner
the y axis is still cutting while the x axis starts cutting before the y is done, leaving a rounded corner on one end and the corner I wanted on the other.
Also sometimes when cutting into wood when the z axis goes down, before it gets a chance to go all the way down to where I programed it the x and y starts cutting leaving unfinished cut wood.
Any help would be great, thanks.

Offline Hood

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Re: Squares not square.
« Reply #1 on: June 20, 2013, 03:15:25 AM »
That is what is called corner rounding and is due to low acceleration/decelleration and is most often associated with machines that have steppers.
Few things you can do if you can not increase the acceleration without losing steps.
 There are settings on General Config page for CV (Constant Velocity) so that on sharp corners CV will be shut off.
You can also do that manually by having Mach run in Exact Stop (G61) mode rather than CV (G64)
Lastly you can code so that the cutter arcs round the corner of the box with the arc being equal to the radius of the cutter.

Do some searching for corner rounding and I am sure you will find lots of information.
Hood

Offline stirling

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Re: Squares not square.
« Reply #2 on: June 20, 2013, 03:20:34 AM »
That would be the effect of CV (so called Constant Velocity Contouring) on a machine where the acceleration is perhaps not as good as you'd like. You can try a few things to mitigate it.

1) Turn CV off.
2) Try to improve your acceleration.
3) Reduce your feedrate
4) Modify CV Distance and CV feedrate on the settings screen (No point in messing with CV angles when you're cutting squares).
 

Offline stirling

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Re: Squares not square.
« Reply #3 on: June 20, 2013, 03:29:04 AM »
Beat me to it there....

That is what is called corner rounding and is due to low acceleration/decelleration and is most often associated with machines that have steppers.

Servo snob  ;D - not if the steppers are driven correctly and within their operating capabilities. Remember I can get as much accel on my stepper router as you have on that big green thing (0.5G right?)  ;).

Ian

Offline Hood

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Re: Squares not square.
« Reply #4 on: June 20, 2013, 03:37:53 AM »
Ha ha not a servo snob at all but its a simple fact that steppers are more prone to this problem.
Regarding your router, go shove 1 to 2 tonne on your gantry and tell me you can still get these accelerations ;)
Hood

Offline stirling

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Re: Squares not square.
« Reply #5 on: June 20, 2013, 04:06:44 AM »
Regarding your router, go shove 1 to 2 tonne on your gantry and tell me you can still get these accelerations ;)
reckon It'd then do 1G... straight down!  ;D

Ian
Re: Squares not square.
« Reply #6 on: June 20, 2013, 10:07:39 AM »
Quote
reckon It'd then do 1G... straight down!

 :D :D

Alex

Offline Hood

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Re: Squares not square.
« Reply #7 on: June 20, 2013, 01:27:15 PM »
;D